In FIG. 1, an air conditioning system 1′ of a motor vehicle, which is known from the prior art, is represented with a refrigerant circuit 2′. The refrigerant circuit 2′ comprises a compressor 3 for compressing the refrigerant, a heat exchanger 4 for cooling and liquefying the compressed gaseous refrigerant at high temperature, an expansion device 5 for lowering the pressure of the refrigerant from a high pressure level to a low pressure level and thus from a high temperature level to a low temperature level, and a heat exchanger 6 for the evaporation and thus for the heat absorption of the two-phase refrigerant present at the lower pressure level and temperature level. The refrigerant which flows, as a vapor, out of the heat exchanger 6, also referred to as evaporator 6, is suctioned by the compressor 3. The refrigerant circuit 2′ is closed. The components of the refrigerant circuit 2′ are connected to one another via refrigerant lines.
If the liquefaction of the refrigerant occurs in the so-called subcritical operation of the refrigerant circuit 2′, as is the case, for example, with the refrigerant R134a, or in certain environmental conditions with the refrigerant R744, which corresponds to the natural refrigerant carbon dioxide, the heat exchanger 4 is referred to as condenser 4. Some of the heat transfer occurs at constant temperature. In the so-called supercritical operation of the refrigerant circuit 2′, or in the case of supercritical heat release in the heat exchanger 4, the temperature of the coolant decreases steadily. In this case, the heat exchanger 4 is also referred to as gas cooler 4. A supercritical operation can occur under certain environmental conditions or operating modes of the refrigerant circuit 2′, for example, with the refrigerant carbon dioxide.
The heat exchanger 4 of the refrigerant circuit 2′ of the air conditioning system 1′ in FIG. 1 is designed as a component of a coolant circuit 7 and thus, on the one hand, refrigerant flows through it, and, on the other hand, a first coolant, for example, a water-glycol mixture, flows through it. The heat exchanger 6 is also designed as a component of a coolant circuit 8, and thus in this case as well, on the one hand, a refrigerant is supplied to it, and, on the other hand, a second coolant, for example, a water-glycol mixture, is supplied to it.
With the air conditioning system 1′, on the one hand, in the heat exchanger 4 operated as a condenser or gas cooler, a heat transfer between the refrigerant and the first coolant is possible, and, on the other hand, in the heat exchanger 6 operated as an evaporator, a heat transfer between the second coolant and the refrigerant is possible.
In the first coolant circuit 7, the heat transferred in the heat exchanger 4 to the coolant is transported to its destination site, which represents a heat sink. The first coolant circuit 7, which receives the heat of the refrigerant, is operated at a high temperature level of the refrigerant and therefore referred to as a high-temperature coolant circuit. In the second coolant circuit 8, heat is transferred from the environment or from a heat reservoir as heat sink to the coolant. The heat is supplied to the refrigerant in the evaporator 6. The second coolant circuit 8, which releases heat to the refrigerant, is operated at a lower temperature level of the refrigerant and therefore referred to as low-temperature coolant circuit.
The air conditioning system 1′ in FIG. 1 can be used as a so-called heating/cooling device, consisting of the refrigerant circuit 2′ and the two independent coolant circuits 7, 8, depending on the operating mode, for heating, for cooling or also for combined heating and cooling in the motor vehicle. Here, it is possible, for example, to heat the passenger compartment and at the same time cool the battery or the power electronics. The refrigerant circuit 2′ is then operated as a water-water heat pump.
Heating/cooling devices designed in this manner are disclosed, for example, in PCT International Application No. WO 2013/023630 A2, European Published Patent Application No. EP 2 629 040 A2, and Japanese Patent Application JP 2010-012949A.
In WO 2013/023630 A2, the heating/cooling device is described as a compact system, wherein, on the one hand, the compressor and the heat exchanger operated as a gas cooler and, on the other hand, the expansion device and the heat exchanger operated as an evaporator are arranged so that they are in thermal contact with one another in each case. The compressor and the heat exchanger operated as a gas cooler are mutually separated from the expansion device and from the heat exchanger operated as an evaporator by a thermal insulation area.
EP 2 629 040 A2 discloses the heating/cooling device with a coolant-refrigerant heat exchanger, which is arranged within the high-temperature coolant circuit and is designed as a plate heat exchanger. The heat exchanger here comprises different areas for the desuperheating and condensation, for the collection and for the supercooling of the refrigerant.
In the heating/cooling devices known from the prior art, in which a first coolant circuit 7 functions as a heat sink for the refrigerant, the desuperheating heat and the condensation heat of the refrigerant, or the heat to be released by the refrigerant at high-temperature level, is transferred in the liquid-cooled heat exchanger 4 to the coolant and conveyed further by the coolant to a heat sink. Depending on the operating mode, one can consider, for example, the air to be supplied to the passenger compartment or the environmental air to be a heat sink.
In the present principle of indirect heat transfer, the efficiency of the overall system is influenced significantly by the return temperature of the coolant. In a design of the heat exchanger 4 as a counter-current heat exchanger, the refrigerant can be cooled in the ideal case to the input temperature of the coolant entering the heat exchanger 4. As the return temperature of the coolant increases, the output temperature of the refrigerant from the heat exchanger 4 consequently also increases.
In particular, in the operation of the air conditioning system in the heat pump mode, at very low temperatures of the environmental air in the range from −15° C. to −20° C., the goal should be to achieve return temperatures of the coolant in the range from approximately 55° C. to 60° C., in order to be able to condition the passenger compartment to comfortable temperatures in the briefest possible time. In this operation, the refrigerant at the outlet of the heat exchanger 4 would ideally have a temperature of more than 55° C.
However, in order to be able to operate the air conditioning system 1′ more efficiently, the refrigerant has to be cooled to temperatures that are much lower than the supply temperature of the coolant.
In the conventional heating/cooling devices, a cooling of the refrigerant to temperatures that are much lower than the supply temperature of the coolant is not possible.
In US 2010/0000713 A1, an air conditioning system for a motor vehicle with a refrigerant circuit and a coolant circuit is disclosed, which are coupled via a heat exchanger for liquefying the refrigerant and thus for the heat release from the refrigerant to the coolant. An inner heat exchanger is located upstream of the heat exchanger in the flow direction of the refrigerant.
Here, inner heat exchanger should be understood to mean a circuit-internal heat exchanger whose function is the heat transfer between the refrigerant at high pressure and the refrigerant at low pressure. In the process, for example, on the one hand, after liquefying, the liquid refrigerant is further cooled or supercooled, and, on the other hand, the refrigerant present in the form of a suction gas is superheated before the inlet of the compressor.
In addition, the air conditioning systems known in the prior art are characterized in that, in the operation in the chiller mode, it takes a very long time to achieve a cooling of the air to be supplied to the passenger compartment via indirect heat transfer from the air to the coolant and subsequently to the refrigerant.
The problem can be solved by providing an air conditioning system with a refrigerant circuit, in which, in the operation in the heat pump mode, additional heat can be discharged from the refrigerant after leaving the heat exchanger operated as a liquefier and used for conditioning the passenger compartment. In addition, in the operation in the chiller mode, the passenger compartment should be cooled in the briefest time possible. It should be possible to operate the air conditioning system with maximum efficiency.